This invention relates to devices for testing the compression of internal combustion engines, and more particularly relates to techniques for compression testing by analyzing the waveform of the starter current or voltage of an internal combustion engine.
Many internal combustion engines have an electrical starting system including a starter motor which draws current from a battery in order to turn or crank the engine until it starts. The starting motor voltage and current waveforms vary in proportion to the work done as the air-fuel mixture in each engine cylinder or chamber is compressed during cranking. The greater the compression produced by a cylinder, the greater the voltage drop and the greater the current flow from the battery to the starter. If a cylinder is leaking, and therefore has low compression, the starter does not need to work as hard in order to compress the air-fuel mixture in the cylinder. As a result, the voltage drop and current flow during the compression stroke of a leaking cylinder is lower than the voltage drop and current flow during the compression stroke of a normal cylinder.
Various devices have been devised in the past to check engine compression using the foregoing principles. For example, one compression tester said to be developed by Mobil Research and Development Corporation produces a waveform on a strip chart corresponding to the current flow in the starter. Before the chart can be used, it must be subjectively analyzed by a highly trained person capable of accurately evaluating the waveforms. This requirement severely limits the acceptability of the chart and increases the expense of using it.
U.S. Pat. No. 3,765,233 (German - Oct. 16, 1973) describes a device in which the percentage of the maximum compression of the engine may be figured out with respect to any cylinder by manually adjusting a potentiometer. However, this process requires considerable time and effort on the part of the operator.
U.S. Pat. No. 3,839,906 (Hanson - Oct. 8, 1975) describes test apparatus which indicates cylinders having a compression below a predetermined reference value. However, the apparatus does not appear to provide any means for indicating the precise relative compression between the various cylinders of an engine.
The present invention overcomes the defects inherent in the prior art devices which rely primarily on analog circuits in order to perform the limited analysis of which they are capable. The present invention employs a novel digital approach in which a group of discrete digital amplitude signals having values proportional to the amplitude of an electrical power signal supplied to the starter of an engine are generated for each chamber of the engine. A processor then selects from each group of digital amplitude signals a representative signal for each chamber of the engine. The representative signals then are analyzed digitally in order to display to the operator the relative compression of the engine chambers.
According to another feature of the invention, timing signals identify the order in which the discrete digital amplitude signals are generated. By using the timing signals, the representative signal for each chamber can represent the maximum value or minimum value of the amplitude signal in each group depending on the order in which the maximum or minimum values are generated. By using this approach, the relative compression of the engine chambers can be evaluated with a degree of accuracy heretofore unavailable. In addition, the processor is capable of performing complex analysis which aids the operator to accurately and efficiently determine the status of the engine compression.
According to a further feature, the number of chambers automatically is determined and the engine ignition then is disabled so that the compression of the chambers can be evaluated during cranking.